The present invention relates to mounts for large bearings and bearings which sustain very high loads, and in particular to bearings which are made from materials that do not readily permit welding of the bearing.
Large material handling units, for example large, mobile cranes or the like, conventionally require the use of a large diameter bearing to rotatably mount an upper material handling assembly on a vehicular base. In the case of a large crane or the like, the unit includes an upper cab or turret on which the lifting boom is mounted, and the turret is rotatably mounted on either a truck chassis or rail car chassis.
Due to the extremely high loads borne by such bearings, the actual load bearing surfaces on the bearing races must be made from a hardenable material such as medium carbon steel. Although such hardenable materials are able to sustain these high loads, these materials are difficult to weld by conventional techniques. The welding of such hardenable materials with conventional methods normally results in an embrittlement or cracking of the metal surrounding the weld. Due to the often microscopic character of such cracking, this condition is difficult to detect by simple inspection techniques. This can severely weaken the strength of a joint formed with such a weld and can lead to a catastrophic failure of the bearing and the crane structure supported thereon. It is possible to weld hardenable materials such as fifty carbon steel with special welding techniques. However, these techniques involve preheating of large steel components to elevated temperatures and this is often impractical.
For these reasons, most large diameter bearings are secured to surrounding structures by bolts or similar fasteners. However, bolt mounting of these bearings can also pose substantial problems, both during and after assembly of the unit. For example, during the initial manufacture of a large mobile crane, all of the bolt receiving apertures must be accessible both from above and below the bearing. This bolt mounting requires a workman to climb beneath or inside the equipment beneath the bearing support area to access the bolt aperture. After assembly, because of the cyclic loading of the crane the bearing mounting bolts can work loose. Often such equipment does not receive proper maintenance and the mounting bolts are allowed to loosen or fall away. Again, this can lead to a catastrpohic failure of the structure. Similar to the problems of initially assembling such bearings with mounting bolts, one reason for the low maintenance of such cranes is the difficulty in gaining access to the mounting bolts within the support base.
Further, after sufficient use the bearing can become worn and must be overhauled or replaced. Other situations arise in which bearings are retrofited onto an existing crane, such as when an old roller bearing assembly is replaced with a newer ball bearing assembly or the like. In such retrofit situations, it is difficult in the field to match the bearing ring bolt apertures to the corresponding support base bolt apertures. Very close tolerances must be maintained on the placement of bolt apertures in the bearing ring if the new or refurbished bearing ring is to fit onto the existing support base.
Due to the above noted problems, practitioners have heretofore attempted to mount such bearings on large cranes or the like by various welding techniques. One such technique requires that the bearing ring itself be heated and maintained at high temperatures throughout the welding process in order to avoid embrittlement in the area surrounding the weld. Another technique involves the welding of the heated bearing ring to a mild steel weld band which is used as an attachment structure for later welding or otherwise suitably mounting the bearing to its support structure. However, such welding techniques can only be practiced in the controlled environment of a factory, and even under these controlled conditions, these techniques are both difficult and expensive. It will be readily apparent that any technique involving heating is very difficult, if not impossible, to conduct upon equipment while in the field.
Another attempt to weld-mount such bearings has required the centrifugal casting of the bearing rings from metals having different chemical compositions, in an attempt to produce a bearing ring having a hardenable composition in the bearing race area but a more ductile and weldable composition in the mounting area. As will be recognized, such a centrifugal casting method greatly increases the cost of the bearing manufacture.
In other applications spacers or pads or the like have been welded to supports beneath bearing rings, but such spacers still require that the bearing ring be secured by bolts to the support base. This type of mounting therefore does not avoid the problems associated with mounting bolts. Heretofore such large diameter, high hardness bearings have not been satisfactorily mounted by welding techniques.